As an air compressor, which is one of typical types of compressors, there are some types, such as an oil-feed type which is configured to inject oil into a compression operation chamber, an oilless type which does not inject oil. Furthermore, the oilless type air compressors also have some types such as a water-injection type and a dry type one; the water-injection type injects water, and the dry type does not. Hereinafter, an oilless type air compressor, including a water-injection type and a dry type one, may be referred to as an oilless compressor.
Although an oilless compressor does not feed oil into a compression operation chamber, it is generally necessary to feed with oil for lubrication into some parts located outside of the compression operation chamber, i.e., a bearing, a drive gear for transmitting power from a power source such as an electric motor, and a timing gear used in, for example, a screw compressor having two or more rotors. Furthermore, in a dry type oilless compressor, a compressor body has a high temperature because of adiabatic compression; therefore, for example, in order to suppress thermal deformation of a housing of the compressor body, there are some dry type oilless compressors that are provided with a cooling jacket around a compression chamber, thus cooling the compression chamber with liquid such as water, coolant, oil or so.
Patent Literature 1 discloses an example of the structure of a lubricating oil system of an oilless compressor. The oilless compressor disclosed in Patent Literature 1 is a screw compressor having female and male rotors, and it is structured to use a gear casing (at the lower part thereof) as an oil sump; the gear casing houses a gear connecting a male rotor shaft, which is a driven shaft, and a drive shaft that drives the male rotor shaft.
The gear casing has a function of accumulating the amount of oil necessary for circulation, and, in addition, is provided with an atmospheric communicating tube for letting the internal pressure escape, thus dropping the pressure toward about the same as the atmospheric pressure when it has become excessively high within the casing. Furthermore, as a structure for communication between the air part in the gear casing and the air part in an ending cover of the compressor, it is structured to equalize the internal pressure between the ending cover and the gear casing.
The screw compressor disclosed in Patent Literature 1 is structured to use the gear casing as an oil sump, thereby making it possible to accumulate the amount of oil needed to be fed to a compressor body, and also to maintain the internal pressure of the gear casing at about the same as the atmospheric pressure, so that the drainage of oil from the compressor body can be performed smoothly, and the circulation and feeding of lubricating oil to machine elements such as bearings and timing gears of the compressor body can be performed properly.